Why we use PNP power transistor in Isolation circuit? ?

The signal from optocoupler is first given to PNP power transistor


before giving it to Relay. Why is it so?Why we use PNP power transistor in Isolation circuit? ?
It is to use the small amount of current from the opto sensor to control a larger current that is needed for the relay operation. Why we use PNP power transistor in Isolation circuit? ?
Opto's typically can't handle alot of current. You use the exterior BJT because of the amount of current that is necessary to switch the relay.





Also, the choice of PNP over NPN is due to switching speed. PNP transistors are typically faster than NPN transistors.





If you pulse an NPN with a squarewave signal and look at the output, it's ';rounded'; at the corners, whereas with typical PNP transistors the signal is not as distorted.
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The choice of PNP versus NPN is probably arbeitrary. The signal level from the optocoupler is probably inadequate to energize the relay, so he power transistor is required as a gain stage.